This invention generally relates to methods and systems for generation of a hydrogen-rich stream, and more particularly to a fuel processor apparatus and method for generating a hydrogen-rich gas stream based on the autothermal cyclic reforming (ACR) process.
Fuel processors convert fossil fuels (such as natural gas and diesel fuel) or bio-fuels to either a hydrogen-rich gas for fuel cells or a pure hydrogen stream. The major subsystem of the fuel processor is the reformer. The reformer converts fuel to a reformate stream, which is a mixture of CO, CO2, H2, steam and hydrocarbons. The shift reactor is almost always used downstream of the reformer reactor. In the shift reactor, CO reacts with steam to produce H2 and CO2.
If the hydrogen-rich gas needs to be processed by a Proton Exchange Membrane (PEM) fuel cell, a CO oxidizer is used downstream of the shift reactor. The CO oxidizer utilizes air to oxidize most of the remaining CO to CO2. If the hydrogen-rich gas needs to be processed by fuel cells other than PEM fuel cell the CO oxidizer is not required. If pure hydrogen needs to be generated, a Pressure Swing Adsorber (PSA) is usually used downstream of the shift reactor.
In prior art the following reforming processes are described: 1) Steam Reforming, 2) Partial Oxidation; 3) Autothermal Reforming; 4) Unmixed Reforming.
In chemical process industries, hydrogen is produced in large quantities using the steam reforming process—where the hydrocarbon fuel, typically natural gas, is reacted with steam in catalyst tubes. Because this reaction is endothermic, a furnace is required to transfer heat from another source, such as a fuel burner, to the catalyst tubes. Consequently, steam reforming is best suited for large chemical plants, which normally are more efficient at larger scales. At a smaller scale, an attractive alternative is autothermal cyclic reforming, which eliminates the requirement of the furnace.
Although various processes have been developed for generating hydrogen-rich gas stream for use in such applications as fuel cells, hydrogen vehicle refueling, and industrial use, each has its own drawbacks. The efficiency with which hydrogen-rich gas is generated by the present invention is far superior to that achieved by previous approaches, which involved heat transfer problems that likely lead to inefficiencies.
One example of an application found to have heat transfer problems is the industrial process known as steam reforming in which hydrogen is produced by passing steam and a hydrocarbon through a nickel catalyst. Steam reforming is typically done at temperatures in a range of 700° C. to about 1000° C. and at pressures in a range of about 1 to about 700 psig. These conditions are too severe for the use of reactor tubes made of mild steel or even stainless steel. A high nickel alloy such as Inconel must be used despite the great cost of such an alloy. Furthermore, heat must be supplied since the reaction is highly endothermic.
To avoid this disadvantage there have been proposals in the art for “adiabatic” steam reforming. In this approach, the heat necessary for the endothermic steam reforming reaction is provided by adding some air to the steam hydrocarbon mixture passing through the reactor. The oxygen in the air reacts with the hydrocarbon, liberating heat.
Unfortunately, however, combustion is an “all or nothing” process. If ignition does not occur, the needed heat is not liberated. If ignition does occur, heat is liberated not throughout the reactor where it is needed but at the point of ignition. Since the heat is not liberated uniformly throughout the reactor, there is again a severe heat transfer problem.
Heat transfer is also a substantial problem in other industrial processes in which packed bed reactors are used to carry out endothermic reactions. Examples of such reactions include but are not limited to the cracking of ammonia to make hydrogen/nitrogen mixtures, the gasification of biomass, the catalytic reforming of petroleum hydrocarbons, and the decomposition of methanol.
U.S. Pat. No. 5,827,496 describes the Unmixed Reforming or Autothermal Cyclic Reforming process which involves generation of a hydrogen rich-gas by cycling air and a mixture of fuel and steam. As explained in prior art, the ACR process includes a reforming step, an air regeneration step and a fuel regeneration step.
During the reforming step, fuel and steam react over the nickel catalyst to produce a reformate stream through conventional steam reforming chemistry. During this reforming step, calcium oxide is converted to calcium carbonate as it captures some of the CO2 formed.
During the air regeneration step, air is passed through the packed bed to oxidize the nickel catalyst. The heat released by the oxidation reaction raises the temperature of the packed bed, and the calcium carbonate is decomposed back to calcium oxide.
During the fuel regeneration step, fuel is introduced to the packed bed, reducing the oxide form of the nickel catalyst back to its elemental form.